In the manufacture of beverage cans, such as aluminum beer cans and soft drink cans, the open-ended can bodies are periodically tested to determine whether they meet manufacturing specifications. In the usual practice, a run of can bodies is removed from the line several times a day and pressure tested. In the past, the testing procedure has consisted of clamping the open-ended can body on a short cylindrical mandrel and air under pressure is introduced through the mandrel into the can body. The operator visually observes when the can bulges under pressure and notes this pressure reading. The testing of lids was done in a similar manner, but requires a fixture separate from that used to test the can-bodies.
With the testing method as used in the past, if the can body was not fully aligned and sealed to the mandrel, the gas pressure could unseat the can body from the mandrel causing the can body to flip out of the clamping fixture. To prevent this, in some cases a bar was extended across the closed end of the can body, but the bar tended to interfere with bulging of the can bottom, so that erroneous readings of pressure were occasionally obtained.